It is often desirable to monitor physiological signals during or after delivery of electrical stimulation to a patient. For example, ECG signals are typically monitored during external cardiac pacing. But the electrical stimulation can produce stimulus artifacts in the monitored signal. To reduce the magnitude of such artifacts, monitoring electrodes are often intentionally placed at a considerable distance from stimulation electrodes. The stimulus artifact is attenuated by distance through the body and has less effect on the measured physiological signals of interest. For example, standard ECG lead placement for monitoring a patient during external cardiac pacing is shown in FIG. 1. Two ECG leads are placed high on the chest near each arm, and a third low on the chest near one leg, all at a considerable distance from the stimulation electrodes. To achieve this distant placement of the monitoring electrodes, the caregiver must typically place five separate electrodes on the patient, and connect separate monitoring cables to each of the ECG electrodes (typically, the stimulation electrodes have cables pre-connected).
An alternative to separate, remotely positioned monitoring electrodes is using the stimulation electrodes for both stimulation and monitoring. Monitoring could, in theory, be done during intervals between stimulation pulses, but a large stimulus artifact is typically present for much of that interval, and thus it has not typically been practical to use the stimulation electrodes for ECG monitoring during pacing. For example, the ECG signal induced by the pacing pulse often occurs within 200 milliseconds subsequent to the pacing pulse and before the artifact has sufficiently attenuated to allow for adequate monitoring.
The stimulus artifact during pacing can make the ECG difficult to interpret, even when the ECG is detected on remotely positioned monitoring electrodes. It is generally required that the ECG be clean enough so that the effectiveness of pacing can be assessed and the stimulus energy adjusted as needed. The stimulus artifact is often so large as to make that assessment difficult or impossible.
Internal pacing involves much lower currents applied directly to the heart and monitoring through the stimulation electrodes has been accomplished in the prior art. The problem of stimulus artifacts in the ECG signal principally arises with external pacing where the higher currents and larger therapy electrodes require the use of separate monitoring electrodes located at a sufficient distance. A similar problem exists during external cardiac defibrillation, but there is less need to detect an ECG signal within milliseconds after the stimulus as is the case with pacing.
Electrical stimulation requires the delivery of an electrical current to the body. Current flows from one stimulation electrode to the other, and often results in the stimulation electrodes becoming polarized following delivery of a stimulation pulse (opposite polarity charge buildup on the two electrodes). A stimulus artifact is seen on the monitoring electrodes both during and after the stimulation pulse, with the amplitude of the artifact typically depending on the position of the monitoring electrode in the electric field created by the difference in potential on the two stimulation electrodes.
Physiological monitoring systems typically amplify the potential difference(s) between monitoring electrodes. The signals of interest are often much smaller in magnitude than the stimulus artifact. Thus, the artifact often masks the physiological signals or saturates the circuitry used for monitoring. Certain applications require acquisition of physiological waveforms during or immediately after the stimulus (e.g., cardiac pacing), and the presence of the stimulus artifact is problematic.
Some efforts have been undertaken to mitigate the effect of the stimulus artifact on the physiological monitoring. Way U.S. Pat. Nos. 4,955,381 and 5,080,099 proposed an electrode assembly in which a separate monitoring electrode was spaced a short distance from the stimulus electrode in a single assembly. Dupelle et al. U.S. application Ser. No. 10/958,987, filed on Oct. 5, 2004 teaches providing a depolarizing current following a stimulus to reduce the polarization artifact. Hauck et al. U.S. Pat. No. 5,330,512 taught a solution for implanted electrodes wherein the stimulus is much lower than for external electrodes, and the implanted leads have a much smaller surface area than pads of external electrodes.